I never played World of Warcraft. As far as Diablo 2 goes...if you want to play it as a single-player RPG, you can. You can cruise through Normal difficulty with fairly minimal grinding, no more than you'd expect from most RPGs. After that, you can go further and play though again on higher difficulties (and yes, you can do that in singleplayer) and get crazy loot and whatnot. But the base singleplayer game of 4-5 acts is extremely solid. You level quickly, collect varied loot, engage a large variety of monsters in varied environments, and basically do everything that you do in Diablo 1 only better and more so and fuck you hamburgerboy, you're an asshole.
You basically seem to be complaining that they took the single-player action RPG, and then added an MMO to the end. You don't have to play the MMO, so shut up.